The present invention relates to a control system for an engine having an air intake passage, and more particularly to an improvement in the detecting means for detecting the amount of air sucked into the engine. This is used as one of the means for detecting the operating state of an engine so as to obtain a more accurate air flow rate measurement signal, thereby controlling the operation of the engine more accurately.
When an engine is electronically controlled, it is necessary to always monitor its operating state. Monitoring means of the operating state, rotating speed detecting means, engine temperature detecting means, exhaust gas temperature detecting means, and throttle opening detecting means of the engine are presented, and as a direct relation to the operating state, measuring means of the air intake flow to the engine is presented.
As measurement detecting means of the air intake flow (used as an operating state detecting means of this engine), a heat type air flow sensor, for example, is used. This sensor is disposed in the intake manifold for supplying combustion air to the engine, has a heater controlled for heating, and is constructed to detect and measure the temperature changes of the heater.
The heater is set so that it is exposed to the air flow in the intake manifold, and the heat dissipating effect is variably controlled by the air flow. Therefore, the temperature of the heater corresponds to the air flow velocity in the intake tube, and the air flow in the intake manifold can be noted by monitoring the varying temperature of the heater.
More particularly, the heater constitutes a temperature sensitive element in which its resistance value varies in accordance with the temperature, and the element is constructed to control the heating electric current controlled in an analog manner with a constant temperature. In this case, since a heat dissipating effect corresponding to the air flow in the manifold is set by the element, the amount of electric current used for heating (heating electric current) increases so as to hold the temperature of the element constant when the air flow is increased. More specifically, the temperature state of the element is detected from the resistance value of the element, the amount of heating electric current to the element is controlled so that the temperature is maintained at a specified value, and the amount of air flowing in the manifold is calculated from the amount of the heating electric current.
However, in such an air flow measuring means, the temperature sensitive element set in an air flow to be measured is constructed to control an electric current controlled in an analog manner in a constant temperature state. When the air flow is, for example, varied 100 times, the heating, electric current value to the element varies approximately twice. Therefore, it is necessary to set offset processing means in a measurement output signal amplifier circuit to employ the measuring means for controlling an engine, and the control circuit thereof becomes complicated.
Further, when an engine is controlled by a microcomputer, it is necessary to convert an analog signal, corresponding to a heating current value from a sensor, into digital data and to supply it to an engine control circuit. Consequently, when the measuring means for obtaining the measurement output signal of such an analog state is employed, a highly accurate A/D converting operation must be executed, and an extremely accurate reference voltage power source for the A/D converter is required.